Sofas are usually composed of a frame provided with cushions and various projecting components such as backrests and armrests. In the most common sofas' design, these components are rigidly attached to the frame.
It is also known in the art to provide a sofa with removable and interchangeable backrests and armrests. These components are for example provided with a downwardly projecting shaft removably inserted into a hole made in the frame or in a support bracket added to the frame for this purpose. However, the components can be installed in only one specific orientation, or otherwise are free to rotate about the axis of the shaft. Neither of these alternatives allow a person sitting on the sofa to releasably lock the angular position of the component so he may rest on it in any desired direction.